Cathodes for high intensity discharge lamps generally comprise a rod or core of a refractory metal, such as tungsten that may include a small percentage of thoria and which has a coil of tungsten or tungsten doped with thoria wound thereabout. The coil, which may be a coiled coil or a coiled, coiled coil, as known in the art, is generally wound upon a mandrel, separated therefrom, and subsequently arranged upon its appropriate core, which is fitted into the coil by virtue of having a diameter slightly less than the interior diameter of the coil. The core and coil are then fixtured and the inner layer of the coil is crimped to the core. By passing a current through the crimp area the form ability of the coil and core material is improved and the coil windings deform and embed slightly in the core, making a mechanical lock with some degree of weld in evidence.
This procedure can achieve temperatures that cause recrystallization of the materials and damages the integrity of the assembly. This damage is known to cause failures in downstream operations as well as in finished lamps. Further, the apparatus presently in use has problems dealing with the smaller cathodes that would be required for lower wattage lamps.
It has been proposed to eliminate some of these problems by winding the coil directly on its appropriate core. Such a solution is shown in U.S. Pat. No. 4,952,841; however, this system requires welding the coil to the core, an extra operation that increases the cost of the cathode and causes recrystallization.